Such an electrical apparatus with terminal screw is generally known and disclosed in for example DE 30 28 958 C2. The known electrical apparatus is implemented as a terminal strip with a housing made of insulation material. The housing features a round shaft with a screw that is disposed therein. The shaft leads to an internal thread of a clamping sleeve by means of which electrical conductors can be connected.
The screw head features a screw driver opening. A screw driver can, by means of its blade, be brought into engagement with the screw driver opening of the screw through the shaft opening, so that the screw can be turned with its threaded shank into the internal thread of the clamping sleeve. Thereby an electrical conductor is clamped that is inserted into the clamping sleeve.
In order to secure the position of a screw that is only partially screwed into the internal thread, axially oriented ribs are provided on the internal side of the shaft. In these locations the internal diameter of the shaft is implemented smaller than the external diameter of the screw head. Consequently the screw head is held nonpositively by means of the ribs in a radial direction, whereby partly an unwanted twisting, caused for example through vibrations, or even the loss of the screw can be prevented.
The users of electrical apparatus with terminal screws demand on the one hand that as part of delivery of the electrical apparatus the screw with its threaded shank is already brought into engagement with the internal thread of the clamping sleeve in order that damage to the threads due to inaccurate alignment of the screw on the internal thread of the clamping sleeve is precluded and an electrical conductor that has been inserted in the clamping sleeve can be clamped immediately without loss of time simply by turning the screw.
On the other hand there is the demand that the screw is brought into engagement with the internal thread only to the degree that also electrical conductors with a maximally acceptable diameter can be inserted by means of the clamping opening into the clamping sleeve. This is however not the case if the threaded shank is brought into engagement with the internal thread beyond a certain depth because the clamping opening is reduced with increasing penetration depth. In this case, for the connection of a thick electrical conductor, the screw has to be turned upward to the extent and the clamping opening has to be opened that the electrical conductor can be inserted by means of the clamping opening into the clamping sleeve. Since however, as a rule, power screw drivers are utilized, the screw is frequently indeed loosened to the point that its threaded shank is no longer brought into engagement with the internal thread. In this case the threaded shank has to be initially aligned again on the internal thread in order to assure that the screw is not screwed in askew and the internal thread is damaged.
Therefore it is of central importance that as part of delivery of the electrical apparatus the thread shank is screwed into the internal thread with such a predetermined engagement depth that also electrical conductors with a maximally acceptable diameter can be inserted into the clamping sleeve by means of the clamping opening.